Research Paper GEOSPHERE Lateral magma flow in mafic sill complexes Craig Magee1, James D. Muirhead2, Alex Karvelas3, Simon P. Holford4, Christopher A.L. Jackson1, Ian D. Bastow1, Nick Schofield5, Carl T.E. Stevenson6, Charlotte McLean7, William McCarthy8, and Olga Shtukert3 GEOSPHERE; v. 12, no. 3 1Basins Research Group, Department of Earth Science and Engineering, Imperial College London, London SW7 2BP, UK 2Department of Geological Sciences, University of Idaho, 875 Perimeter Drive, Moscow, Idaho 83844, USA doi:10.1130/GES01256.1 3Schlumberger Multiclient, Schlumberger House, Buckingham Gate, West Sussex RH 6 0NZ, UK 4Australian School of Petroleum, University of Adelaide, North Terrace, Adelaide SA 5005, Australia 5Department of Geology and Petroleum Geology, School of Geosciences, University of Aberdeen, Meston Building, Old Aberdeen, Aberdeen AB24 3UE, UK 17 figures; 1 table 6School of Geography, Earth and Environmental Sciences, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK 7School of Geographical and Earth Sciences, University of Glasgow, Gregory Building, Glasgow G12 8QQ, UK CORRESPONDENCE: c.magee@ imperial .ac .uk 8Department of Earth Sciences, University of St Andrews, Irvine Building, St Andrews KY16 9AL, UK CITATION: Magee, C., Muirhead, J.D., Karvelas, A., Holford, S.P., Jackson, C.A.L., Bastow, I.D., Scho­ field, N., Stevenson, C.T.E., McLean, C., McCarthy, ABSTRACT 2005; White et al., 2008; Ebinger et al., 2010); (2) total melt volume estimates, W., and Shtukert, O., 2016, Lateral magma flow which can be used to assess the thermomechanical state of the mantle (White in mafic sill complexes: Geosphere, v. 12, no. 3, The structure of upper crustal magma plumbing systems controls the dis- et al., 2008; Ferguson et al., 2010); (3) the physiochemical evolution of magma p. 809–841, doi:10.1130/GES01256.1. tribution of volcanism and influences tectonic processes. However, delineating (e.g., Holness and Humphreys, 2003; Cashman and Sparks, 2013); (4) host-rock the structure and volume of plumbing systems is difficult because (1) active in- deformation induced by magma intrusion, which may be expressed at the Received 26 August 2015 Revision received 29 January 2016 trusion networks cannot be directly accessed; (2) field outcrops are commonly Earth’s surface (e.g., Wright et al., 2006; Biggs et al., 2009; Pagli et al., 2012; Accepted 10 March 2016 limited; and (3) geophysical data imaging the subsurface are restricted in areal Jackson et al., 2013; Magee et al., 2013a; Tibaldi, 2015); and (5) volcanic erup- Published online 28 April 2016 extent and resolution. This has led to models involving the vertical transfer tion locations and styles (e.g., Abebe et al., 2007; Gaffney et al., 2007; Mazza- of magma via dikes, extending from a melt source to overlying reservoirs rini, 2007; Tibaldi, 2015; Muirhead et al. 2016). However, the inaccessibility of and eruption sites, being favored in the volcanic literature. However, while active plumbing systems and limitations in exposure at the Earth’s surface there is a wealth of evidence to support the occurrence of dike-dominated makes reconstructing the geometry and connectivity of regionally extensive systems, we synthesize field- and seismic reflection–based observations and intrusion networks from field outcrops challenging. highlight that extensive lateral magma transport (as much as 4100 km) may In order to delimit plumbing systems, it is therefore important to inte- occur within mafic sill complexes. Most of these mafic sill complexes occur grate field observations with data and imaging techniques that probe magma in sedimentary basins (e.g., the Karoo Basin, South Africa), although some in- pathways and storage in the subsurface (Tibaldi, 2015). Indirect analytical ap- trude crystalline continental crust (e.g., the Yilgarn craton, Australia), and con- proaches commonly involve: (1) the examination of ground deformation pat- sist of interconnected sills and inclined sheets. Sill complex emplacement is terns (e.g., using InSAR, interferometric synthetic aperture radar) inferred to largely controlled by host-rock lithology and structure and the state of stress. relate to the emplacement of magma (e.g., Pedersen, 2004; Wright et al., 2006; We argue that plumbing systems need not be dominated by dikes and that Pagli et al., 2012; Sparks et al., 2012); (2) scrutiny of petrological and geochem- magma can be transported within widespread sill complexes, promoting the ical data to assess magma contamination, residence times, crystallization his- development of volcanoes that do not overlie the melt source. However, the tories, and melt source conditions (Cashman and Sparks, 2013, and references OLD G extent to which active volcanic systems and rifted margins are underlain by therein); and/or (3) mapping the location and crude geometry of crystallized sill complexes remains poorly constrained, despite important implications for intrusions or present-day zones of melt using geophysical techniques such elucidating magmatic processes, melt volumes, and melt sources. as potential field, magnetotellurics, and seismicity (e.g., Cornwell et al., 2006; Whaler and Hautot, 2006; Desissa et al., 2013). For example, the application OPEN ACCESS and synthesis of these techniques in the East African Rift system and Iceland INTRODUCTION have greatly improved our understanding of magma-tectonic interactions and allowed processes controlling continental break-up, such as dike intrusion, to Interactions between tectonic and magmatic processes, as well as the be analyzed in real time (e.g., Ebinger et al., 2010; Gudmundsson et al., 2014; lithol ogy and preexisting structure of the host rock, control the geometry and Sigmundsson et al., 2015). It is, however, important to consider that these data connectivity of magma conduits and reservoirs (i.e., a magma plumbing sys- poorly constrain intrusion geometries and are commonly interpreted within This paper is published under the terms of the tem). Delineating plumbing system structure can therefore provide important the classical framework of igneous geology; i.e., magma migration within the CC­BY license. insights into: (1) continental rifting (e.g., Ebinger and Casey, 2001; Kendall et al., Earth’s crust is generally facilitated by the vertical intrusion of dikes, extending © 2016 The Authors GEOSPHERE | Volume 12 | Number 3 Magee et al. | Lateral magma flow in mafic sill complexes 809 Research Paper from a deep magma reservoir or melt source to overlying shallow-level intru- Mountain laccoliths, Utah, USA, Johnson and Pollard, 1973; Torres del Paine sions and volcanoes (Fig. 1A; Tibaldi, 2015, and references therein). laccolith, Chile, Michel et al., 2008), which commonly form single or com- While numerous studies document vertical magma transport in dike-domi- posite, thick (typically >1 km) laccoliths and plutons with limited lateral extents nated volcanic plumbing systems (e.g., Kendall et al., 2005; Keir et al., 2006; ( 20 km), differs significantly from that of mafic sill complexes (e.g., Cruden Wright et al., 2006; Corti, 2009; Bastow et al., 2010; Cashman and Sparks, and McCaffrey, 2001; Corazzato and Groppelli, 2004; Menand, 2008; Bunger ≲ 2013; Muirhead et al., 2015; Tibaldi, 2015), it is also widely accepted that lateral and Cruden, 2011). Mapping of flow indicators within sill complexes reveals magma flow can occur either within individual plumbing systems where vol- that interconnected networks of mafic sills and inclined sheets can facilitate canoes may be laterally offset by as much as ~20 km from their magma reser- magma transport over distances of as much as 12 km vertically and 4100 km voir (e.g., Fig. 1A; e.g., Curtis, 1968; Cartwright and Hansen, 2006; Gudmunds- laterally (Cartwright and Hansen, 2006; Leat, 2008) and could thus feed vol- son, 2006; Aoki et al., 2013; Aizawa et al., 2014; Muirhead et al., 2014; Tibaldi, canic eruptions that are laterally offset considerable distances from the source 2015) or giant radiating dike swarms, which can facilitate lateral magma flow of mantle or lower crustal melting (Fig. 1B; Magee et al., 2013b; Muirhead over hundreds to thousands of kilometers (e.g., Ernst and Baragar, 1992; Ernst et al., 2014). The consideration that a plumbing system may be characterized et al., 1995; Macdonald et al., 2010). Recent field-, seismic reflection–, and by a laterally extensive sill complex, as opposed to a network of vertical dikes, modeling-based research further indicate that plumbing systems at shallow could therefore have broad implications for the interpretation of geochemi- levels (typically <3 km depth), particularly those hosted in sedimentary basins, cal, petrological, and geophysical data obtained from volcanic settings (e.g., may instead be characterized by extensive sill complexes (e.g., Chevallier and Meade et al., 2009). For example, recent field- and seismic-based studies have Woodford, 1999; Smallwood and Maresh, 2002; Thomson and Hutton, 2004; suggested that surface deformation, generated by sill and sill complex em- Planke et al., 2005; Cartwright and Hansen, 2006; Kavanagh et al., 2006, 2015; placement, may not directly correlate to the location and volume of intruding Lee et al., 2006; Leuthold et al., 2012; Leat, 2008; Menand, 2008; Polteau et al., magma if plastic deformation of the host rock occurs (Jackson et al., 2013; 2008a; Cukur et al., 2010; Bunger and Cruden, 2011; Gudmundsson and Løtveit, Magee et al., 2013a, 2013b; Schofield et al., 2014). 2012; Muirhead